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Two Marks Q&A
UNIT I 1. What are the transistor parameters that vary with the
temperature?
,ICO,VBeo are the parameters varying with the temperature.
2. What is Bias? What is the need for biasing?
The proper flow of zero signal collector current and the
maintenance of proper collector emitter voltage during the passage
of signal is know as
transistor biasing.
When a transistor is biased properly, it works efficiently and
produces no distortion in the output signal and thus operating
point can be maintained
stable. 3. What do you understand by DC & AC load line?
DC Load Line
It is the line on the output characteristics of a transistor
circuit which gives the values of Ic & Vce corresponding to
zero signal (or) DC Conditions.
AC Load Line
This is the line on the output characteristics of a transistor
circuit which
gives the values of Ic & Vce when signal is applied. 4. What
is the meant by operating point Q?
The zero signal values of Ic & Vce are known as operating
point. It is also called so because the variations of Ic and Vce
take place about this point, when the signal is applied.
5. What are the types of biasing?
The different types of biasing are
(i) Fixed bias
(ii) Collector to Base bias (or) Feedback bias
(iii) Self bias (or) Voltage divider bias 6. What are all the
factors that affect the stability of the operating point?
The following are the factors that affect the stability of the
operating point,
a. Change of due to replacement of transistors.
b. Thermal variations 7. Define stability factor S?
The stability factor is defined as the rate of change of
collector current Ic
with respect to the reverse saturation collector current Ico,
keeping Vbe and constant.
8. What are the disadvantages of collector feedback bias?
The disadvantages of feedback bias are a. The collector current
is high.
b. If AC signal voltage gain feedback into the resistor Re, it
will reduce the gain of the amplifier.
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9. Why voltage divider bias is commonly used in amplifier
circuit?
The voltage divider bias has the following advantages a. the
operating point will be in stable position. b. The stability will
be considerably improved. c. Ic can be reduced to the collector
leakage current ICO.
10.Define the stability factors S and S ? The Stability factor S
is defined as the rate of change of Ic with VBE
keeping ICO and constant.
The Stability factor S is defined as the rate of change of Ic
with keeping VBE and ICO constant.
11.Give the stability factor S for the fixed bias circuit.
The stability factor for the fixed bias circuit is,
12.Give the stability factor S for the Collector to base bias
circuit.
The stability factor for the Collector to base bias circuit
is,
13.Give the stability factor S for the Voltage divider bias
circuit.
The stability factor for the Voltage divider
bias circuit is,
14. Why fixed bias circuit is not used in practice?
The stability of the fixed bias circuit is very less. Since the
stability factor S = 1+ , is a large quantity, therefore stability
is less. So, it is not used in amplifier circuits.
15. What are all the compensation techniques used for bias
stability?
Along with the negative feedback, the following techniques are
used for the Q point stability.
a. Diode compensation,
b. Thermistor compensation,
c. Sensistor compensation.
16.Why the input impedance of FET is more than that of a BJT?
The input impendence of FET is more than that of a BJT because the
input
circuit of FET is reverse biased whereas the input circuit of
BJT is forward
biased. 17.How FET is known as Voltage variable resistor?
In the region before pinch off, where VDS si small, the drain to
source
resistance rd can be controlled by the bias voltage VGS.
Therefore FET is useful
as voltage variable resistor (VVR) or Voltage dependent Resistor
(VDR)
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18.List the advantages of Fixed bias method?
The advantages of fixed bias method are, a. The stability of the
operating point is greatly improved
when compared with the other circuits.
b. Less cost and simple circuit. 19.How self-bias circuit is
used as constant current source?
In the self bias circuit if Ic tends to increase because of ICO
has increasing
as a result of temperature, the current in RE increases. As a
consequences of the increase in voltage drop across RE that
provides negative feedback, the
base current is decreased. Hence constant IC value is maintained
in the self bias circuit
20.What is Thermal runaway?
The continuous increase in collector current due to poor biasing
cause the temperature at collector terminal to increase. If no
stabilization is done, the collector leakage current also
increases. This further increases the temperature. This action
becomes cumulative and ultimately the transistor burns out. The
self destruction of an unstabilised transistor is known as thermal
runaway.
21. What are the consideration factors that are used for the
selection of an
operating point for an FET amplifier?
The consideration factors are, a. Output voltage swing, b.
Distortion, c. Power dissipation, d. Voltage gain,
e. Drift (or) Drain current. 22.Write the different types of FET
biasing circuits.
The FET biasing circuits are classified as,
a. Gate bias, b. Self bias
c. Voltage divider bias,
d. Current source bias
e. Drain feedback bias f. Zero bias.
23. What is meant by stabilization?
The maintenance of the operating point fixed stable is known as
stabilization.
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UNIT II
1. What is an amplifier?
An amplifier is a circuit, which can be used to increase the
amplitude of
the input current or voltage at the output by means of energy
drawn from an external source.
2. Based on the transistor configuration how amplifiers are
classified.
Based on transistor configuration, the amplifier are classified
as
a. Common Emitter amplifier
b. Common Collector amplifier c. Common Base amplifier
3. Draw a CE amplifier & its hybrid equivalent circuit.
4. Draw a CC amplifier & its hybrid equivalent circuit
5. Draw a CB amplifier & its hybrid equivalent circuit
6. Write the Hybrid parameters equation for transistor
amplifier?
Vi = hi Ii + hrVo Io = hf Ii + hoVo
7. Write the CE amplifier Current gain, Voltage gain, Input
Impedance,
Output Impedance in terms of h-parameters.
Current gain Ai = -hfe Voltage gain Av = ( - hfeRL)/hie Input
Impedance Zi = hie Output Impedance Zo = ( hfe+Rs)/(hoeRs + h)
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8.Write the current amplification factors of the three
transistor amplifier
configurations. In a transistor amplifier with AC input signal,
the ratio of change in output
current to the change in input current is known as the current
amplification factor.
9. Which amplifier is called as voltage follower? Why?
The common collector transistor amplifier configuration is
called as voltage follower. Since it has unity voltage gain and
because of its very high input impedance. It doesnt draw any input
current from the signal. So, the input signal is coupled to the
output circuit without making any distortion.
10.Why hybrid parameters are called so? Define them?
The dimensions of the hybrid parameters are not alike, that is
they are hybrid in nature so they are called hybrid parameters.
h11 = [ V1/I1] at V2=0; h11 = Input impedance with output port
short circuited. h12 = [ V1/V2] at I1=0; h12 = Reverse voltage gain
with input port open circuited.
h21 = [ I2/I1] at V2=0;h11 = Forward current gain with output
port short circuited. h22 = [ I2/V2] at I1=0; h11 = output
impedance with input port open circuited.
11.What are the salient features of hybrid parameters?
The salient features of hybrid parameters are, a. h parameters
are real numbers,
b. They are easy to measure. c. They are convenient to use in
circuit analysis and design d. Easily convertible from one
configuration to other
e. Readily supplied by manufactures.
12. Write the input impedance, output impedance, voltage gain
and current gain
of the common emitter amplifier in terms of h parameters for the
fixed bias
condition? Current gain Ai = -hfe Voltage gain Av = (hfeRC)/hie
Input Impedance Zi = hie Output Impedance Zo = RL ||RC
13. Define Miller effect in input capacitance?
For any inverting amplifier, the input capacitance will be
increased by a miller effect capacitance, sensitive to the gain of
the amplifier and the inter
electrode capacitance connected between the input and output
terminals of
the active device. CMi = (1-Av)Cf ; CMo=Cf Cf = Inter electrode
capacitance between input and output
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14.What is a Darlington connection in the amplifiers?
A Darlington transistor connection provides a transistor having
a
very large current gain, typically a few thousand. The main
features of the Darlington connection is that the composite
transistor acts as asingle unit
with a current gain, that is the product of current gains of the
individual
transistors.
D= 1 2
D = Darlington connection current gain
1 and 2 Current gain of the transistors 1 & 2 in the
Darlington pair
16.Draw the darlington connections using similar transistors
17.Draw the Darlington connection using complementary
transistors.
18.What does bootstrapping mean?
In Darlington transistor pair circuits, the input impedance is
reduced because of the biasing resistors in the circuit. To
overcome this, decrease in the input resistance due to the biasing
network, a small capacitor and resistance R3 are added in the
circuit. This improved the
input impedance of the darlington pair circuit. C is added at
the input side and R3 is connected between output
and input circuits. Reff = Effective input resistance.
Reff = R3/(1-Av) Reff = Large value Av= Voltage gain.
19.Why we go for differential amplifier? (or) What is the need
of differential
amplifier? The need for differential amplifier arises in many
physical
measurements, in medical electronics and in direct coupled
amplifier applications. In this amplifier, there will be no output
voltage resulting from thermal drifts or any other changes
provided, changes in both halves of the circuits are equal.
20.Define Common Mode Rejection Ratio CMRR?
Common Mode Rejection Ratio is the figure of merit of a
differential
amplifier and is given by,
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21. What are the advantages of differential amplifier?
The advantages of differential amplifier are, a. Very stable
b. Low noise, low drift,
c. Variations in supply voltage, temperature etc., will not
change
the gain of the amplitude. d. Does not require any coupling
capacitor. e. Frequency response is better.
22.What are the applications of a differential amplifier?
The applications of a differential amplifier are,
a. To measure many physical quantities, b. Can be used as a
direct coupled amplifier, c. Used in operational amplifier.
23.What does bootstrapping technique mean?
If one end of the resistor changes in voltage, the other end of
the
resistor also moves through the same change in voltage. This
technique
is known as bootstrapping. It is used to increase the input
impedance of
the darlington pair circuits.
24.What are the limitations of h parameters?
The h parameters has the following limitations, a. The accurate
calculation of h parameters is difficult. b. A transistor behaves
as a two port network for small signals
only, hence h parameters can be used to analyze only the small
signal amplifiers.
25.What is meant by unity gain frequency?
The frequency at which the gain approaches unity is known as
unity gain frequency.
26. Define dynamic range of an amplifier?
Dynamic range of an amplifier is defined as the range over
which
an active electronic device can produce a suitable output signal
in
response to an input signal. 27.What is a differential
amplifier?
An amplifier that has two inputs and produces an output signal
that
is a function of the difference between the two given inputs is
called differential amplifier.
28.Why transformer coupling is not used in the initial stages of
a multistage amplifier?
The transformer-coupled amplifiers are not used in the initial
stages
of a multistage amplifier because it produces unwanted noise.
Once these
signals are amplified, it cannot be eliminated by the other
stages hence the amplifier performance is deteriorates.
29.Write the need for constant current source for difference
amplifier?
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The necessary for constant current source for differential
amplifier
to increase the common mode rejection ratio without changing the
quiescent current and without lowering the forward current
gain.
30. Why constant current source biasing is preferred for
differential amplifier?
The constant current source biasing is preferred for
differential
amplifier in order to increase the input resistance and to make
the
common mode gain is zero. 31. What is Common mode voltage
swing?
The common mode voltage swing is defined as the maximum peak
input voltage which may be applied to either the input terminal
without
causing abnormal operation or damage. Typically with power
supplies of +6v or 6V, the common mode voltage swing should not
exceed +2v
32.Why Re is replaced by a constant current bias in a
differential amplifier?
The emitter supply VBE used for biasing purpose must become
larger as Re is increased in order to maintain the quiescent
current at its proper value. If the operating currents of the
transistors are allowed to decrease, this will lead to higher hie
values and will tend to decrease
CMRR. To overcome this practical limitations RE is replaced by a
constant
current bias.
33.What is the input impedance of differential amplifier with RE
at its emitter junction?
The input impedance of a differential amplifier RL at its
emitter
junction is RL= 2hie+(1+hfe)RE
34. What are the special features of a difference amplifier that
used FETs? The special features of difference amplifier using FET
are, a.Very high input impedance. b. The common mode rejection
ability is increased which makes the
common gain almost zero. 35. State the various methods of
improving CMRR?
The methods of improve CMRR are, a. Constant current bias method
b. Use of current mirror circuit c. Use of active load
36.Write the two types linear differential amplifiers?
The linear differential amplifiers are classified as, a.
Inverting amplifier.
b. Non Inverting amplifier
37.Write the gain equation of inverting amplifier?
The gain equation of inverting amplifier is, Avf = Vo/Vin =
-(Rf/Ri)
Avf= Gain with feedback
Rf = Feedback path resistor Ri = Input resistor.
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38.Write the configuration of differential amplifier?
The differential amplifier has the following configurations, a.
Dual input, balanced output differential amplifier,
b. Dual input, Unbalanced output differential amplifier,
c. Single input, balanced output differential amplifier, d.
Single input, unbalanced output differential amplifier,
39.What do you mean by bisected network?
Consider of a particular network, which has mirror symmetry
with
respect to an imaginary line. If the entire network is denoted
as N, then it can be divided into two half networks N/2 about the
line of symmetry
as shown below.
The two half networks can be connected using any number of
wires
but the wires are not crossing. This type of network is known as
bisected
network. 40.Give the condition for analyzing the simplified
Hybrid model of the
transistor amplifier? The following condition should be
satisfied for analyzing the simplified
hybrid model of transistor amplifier. hoe.RL
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UNIT III
1. Define the frequency response of Amplifier?
The frequency response of an amplifier can be defined as the
variation of output of quantity with respect to input signal
frequency. In
otherwise it can be defined as a graph drawn between the input
frequency and the gain of an amplifier.
2. Define lower & upper cut off frequencies of an
amplifier.
Lower cut-off frequency
The frequency (on lower side) at which the voltage gain of
the
amplifier is exactly 70.0% of the maximum gain is known as lower
cut off frequency.
Upper cut-off frequency
The frequency (on higher side) at which the voltage gain of the
amplifier is exactly 70.0% of the maximum gain is known as upper
cut off
frequency. 3. Define bandwidth?
The range of frequencies occupied by the signal is known as
its
bandwidth. 4. State the reason for fall in gain at low
frequencies.
The coupling capacitance has very high reactance at low
frequency. Therefore it will allow only a small part of signal from
one stage to next stage and in addition to that the bypass
capacitor cannot bypass or shunt the emitter resistor effectively.
As a result of these factors, the voltage gain rolls of at low
frequency.
5. State the reason for fall in gain at higher frequencies?
At high frequency the reactance of coupling capacitor is very
low. Therefore it behaves like a short circuit. As a result of this
the loading
effect of the next stage increase which reduces the voltage
gain. Hence
the voltage gain rolls off at high frequencies. 6. Why the
electrolytic capacitor is not used for coupling?
Electrolytic capacitor is a polarized capacitor. So it cannot be
used
for coupling and also in electrolytic capacitor, the dielectric
is not an insulating material but it conducting material which will
change the
capacitance effect.
7. Write a note on effects of coupling capacitor.
a. The coupling capacitor Co transmits AC Signal. But blocks Dc.
This
prevents DC interferences between various stages and the
shifting of operating point.
b. It prevents the loading effect between adjacent stages.
8. Draw the low frequency simplified h-parameter model of an
amplifier with
a unbypassed emitter resistor.
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10.Why an NPN transistor has a better high frequency response
than the
PNP transistor? An NPN transistor has a better frequency
response than the PNP
transistor because the mobility of electron is more and
capacitive effect is
less. 11.Write an expression for the bandwidth of multistage
amplifier.
The bandwidth of multistage amplifier is f2 f1 = fo (21/n-1)
12.What is the significance of gain bandwidth product?
It is very helpful in the preliminary design of a multistage
wideband amplifier. This can be used to setup a tentative circuit,
which is often used for this purpose.
13.Why is the gain bandwidth product a constant?
It is defined as the magnitudes of the product of the mid band
gain which is a constant and the bandwidth, which is also a
constant. Hence the product of two constants should also be a
constant.
14. Why N-channel FETs have a better response than P-channel
FETs?
N-channel FET have a better high frequency response than P-
channel FET due to the following reason.
a. Mobility of electrons is large in N-channel FET whereas the
mobility of holes is poor in P-channel FET.
b. The input noise is less in N-channel FET that that of the
P-channel
FET. c. The trans conductance is larger in N-channel FET that
that of P-
channel Fet.
15.Define f T and f . Unity gain frequency (f T) or frequency
parameter.
It is defined as the frequency at which the common emitter
shirt
circuit current gain has dropped to unity and is denoted by the
symbol
(fT) Beta cut-off frequency (f )
It is defined as the high frequency at which -of a CE
transistor
drops to 0.707 or 3dB from its lower frequencies 16.What is the
need for having a high value of fT?
Bandwidth of the amplifier is directly proportional to fT. hence
tp have larger bandwidth, the value of fT should be high.
17.What is a cascade amplifier?
The cascade configuration is an amplifier stage composed of a
direct coupled common emitter / common base combination. This
offers
the possibility of a very large bandwidth. 18.Write the relation
between the bandwidth and rise time of an amplifiers?
BW=fH = 0.35/tr
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19.Write the overall lower cut off frequency of multistage
amplifier?
fL(n) = f L / ( (21/n-1))
Where f L(n) lower 3 dB frequency of identical cascaded stages n
- Number of stages
20. Write the overall higher cut off frequency of multistage
amplifier?
FH(n) = f H ( (21/n-1))
Where f H(n) lower 3 dB frequency of identical cascaded stages n
- Number of stages
21.Write the relation between the sag and lower cut-off
frequency. The tilt of sag in time t1 is given by fL = pf / 100
p= Y of tilt
f= input signal frequency 22.For an amplifier, midband gain is
100 and lower cut off frequency is 1kHz. Find the gain of an
amplifier at the frequency of 20Hz.
A = (Amid)/( (1+(f1/f2)2))
A = (100)/( (1+(1000/20)2)) = 2
23.Write the relation between the sag and lower cutoff
frequency?
fH = 2.2 / (2 tr) = 0.35/tr 24. What is dominant network?
In high frequency analysis of an amplifier, the network having
lower critical frequency is called dominant network.
25.What is the function of Miller input capacitance of an
amplifier?
The Miller input capacitance of an amplifier is a function of
Bypass
capacitor.
26.What is the use of source bypass capacitor in CS
amplifier?
Source bypass capacitor in CS amplifier is used for improving
the
voltage gain. 27.Give two advantages of common source FET
amplifier?
a. Good voltage gain
b. High input impedance. 28.What are the advantages of
representation of gain in decibels?
a. In multistage amplifier, it permits to add individual gains
of the
stages to calculate overall gain.
b. It allows us to denote, both very small as well as very large
quantities of linear scale by considerably small figures.
29.What is the coupling methods used for coupling in multistage
amplifiers?
The coupling methods used are,
a. RC coupling b. Transformer coupling
c. Direct coupling
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UNIT IV
1. List out the classification of large signal amplifiers? The
large signal amplifiers are classified as follows.
a. Based on the input i. small signal amplifiers ii. large
signal amplifiers
b. Based on the output
I. Voltage amplifier II. Power amplifier
III. Current amplifier
c. Based on the transistor configuration I. CE amplifier
II. CB amplifier
III. CC amplifier
d. Based on the number of stages
I. Single stage amplifier II. Multistage amplifier
e. Based on the Bandwidth I. Untuned amplifier (wide band
amplifier)
II. Tuned amplifier (narrow band amplifier)
f. Based on the frequency response
I. AF (Audio frequency) amplifier
II. IF (Intermediate frequency) amplifier III. RF (Radio
Frequency) amplifier
g. Based on the Biasing condition
I. Class A amplifier
II. Class B amplifier III. Class C amplifier
IV. Class AB amplifier
V. Class D amplifier VI. Class S amplifier
2. How do you bias the class A operation?
In class A mode, the output current flows through out the entire
period of input cycle and the Q point is chosen at the midpoint of
AC
load line and biased.
3. Which amplifier gives minimum distortion? Class S amplifier
gives minimum distortion.
4. Give the applications of class C power amplifier.
The applications of class C power amplifier are,
a. Used in radio and TV transmitters.
b. Used to amplify the high frequency signals. c. Tuned
amplifiers
5. Give the two draw backs of class C amplifier.
The drawbacks of class C amplifier are,
a. Distortion is high.
b. Figure of merit is low.
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6. What is Class D amplifier?
In order to increase the conversion efficiency, it would be
desirable to make the device to operate as a switch. So that its
voltage
drop remains almost at minimum value over the half cycle of
output
current flow. Such a system is called class D amplifier.
9. State the merits of using push pull configuration.
The merits of push pull configurations are,
a. Efficiency is high (78.5%) b. Figure of merit is high c.
Distortion is less.
d. Ripple present in the output due to power supply is
multiplied.
10.What are the advantages of using complementary symmetry
configuration? The advantages of using complementary symmetry
configuration are,
a. It does not use centre-tapped transformer either at input
or
output.
b. It uses one PNP transistor and one NPN transistor hence it
provides
proper impedance matching. Hence its voltage gain is unity.
11.Define conversion efficiency of a power stage.
The ratio of the AC output power delivered to the load to DC
input
power applied is referred to as conversion efficiency. It is
also called as
collector circuit efficiency in case of transistor
amplifier.
Signal power delivered to the load x 100 = DC Power supplied at
input circuit
12.Write down the values of maximum possible power conversion
efficiency
for class A direct coupled and transformer coupled. For class A
direct coupled = 25%
For class A transformer coupled = 50%
13. What is cross over distortion?
In class B mode both transistors are biased at cut off
region
because the DC bias voltage is zero. So input signal should
exceed the barrier voltage to make the transistor conduct.
Otherwise the transistor
doesnt conduct. So there is a time interval between positive and
negative alternations of the input signal when neither transistor
is conducting. The resulting distortion in the output signal is
cross over distortion.
14.How cross over distortion is eliminated?
To avoid cross over distortion, a light forward bias (0.3Vfor ge
0.6V for
Si) voltage is applied to the emitter junction of both the
transistors. It
causes transistor to conduct immediately when the input signal
is applied. So Q point is fixed above cut off.
15.What are the drawbacks of the transformer coupled
amplifier?
The disadvantages of transformer-coupled amplifiers are,
a. Transformer are bulky b. Loss is more
c. Centre-tapped of transformer is difficult. 16. Why RC
coupling is popular?
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RC coupling is popular because it is simple, less expensive,
less
distortion and it provides uniform bandwidth.
17.List the advantages of transformer coupled amplifier.
The advantages of transformer coupled amplifier are, a. it is
more efficient because the low DC resistance of the primary is
connected to the collector circuit. b. It provides excellence
impedance matching, thus voltage and power
gains are improved.
18.What is the use of transformer coupling in the output stage
of multistage amplifier?
The transformer coupling provides impedance matching between
input and output. As a result the power gain is improved.
19.State the reason for fall in gain at low frequencies in the
RC coupled
amplifier. a. The coupling capacitance (input) has very
reactance at low
frequency. Therefore it will allow only a small part signal from
one stage to next stage.
b. The bypass capacitor cannot bypass or shunt the emitter
resistor effectively.
As a result of these factors, the voltage gain rolls off at low
frequency.
20.State the reason for fall in gain at high frequencies. At
high frequency, the reactance of coupling capacitor (output) is
very low. Therefore it provides like a short circuit. As a
result of this, the loading effect of the next stage increase which
reduces the voltage gain. Hence the voltage gain falls off at high
frequencies.
21.What is amplitude or non linear distortion?
In case of large signal amplifier, the input signals are large
in amplitude. So the operation extends to non linear region of its
transfers characteristics. Of the signal excursion enters the
non-linear region then distortion occurs in the output. Such a
distortion is called non-linear distortion.
22.Define figure of merit. Figure of merit is defined as the
ratio of maximum collector current
dissipation power to the maximum AC power developed across the
load. 23.Where S amplifiers are used?
The class S power amplifier can be used to amplify either
the
constant amplitude or varying amplitude signal such as FM or AM
signal. 24. Define inter modulation distortion?
Inter modulation distortion is a type of non-linear distortion.
Which
generate frequency components not harmonically related to the
signal
frequencies. It occurs when the input signal contains more than
the one
frequency.
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25.What is the use of heat sink?
The heat sink is used to observe the heat produce in the
transistor
junctions while its operation. Usually power amplifiers are
provided with
heat sinks. The heat sink is a large, black metallic
heat-conducting device placed in close contact with the
transistor.
26.What is the maximum power dissipation hyperbola?
Maximum power dissipation hyperbola represents focus of all the
points at which the collector dissipation is exactly Pc Max . The Q
point
must lie on or below the hyperbola for safe operation.
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27. Define thermal resistance.
The resistance offered by the bipolar junction transistor to
the
flow of heat is called thermal resistance. The thermal
resistance Q = QjA = QjC+QCS+QSA
o C/W QjA = Total junction to ambient thermal resistance QjC=
Junction to case thermal resistance. QCS= Case to heat sink
resistance. QSA=Heat sink to ambient resistance.
28.Write the maximum power handling of the class C power
amplifier? The maximum power in class C power amplifier is,
Pc max = 5/12 (T/To)(Vcc2/RL)
29.Write the advantages of heat sink?
The advantages of heat sink are,
a. The temperature of the case gets lowered.
b. The power handling capacity of the transistors can approach
the
rapid maximum value. 30.Write the derating factor expression of
a power transistor?
Pd(T1) = Pd(To) (T1 - To) WherePd(To) = Rated power handling
capacity at To
Pd(T1) = Power handling capacity at T1 The derating factor is
expressed in the units of watts / degree of
temperature or milliwatts /degree of temperature.
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31.What is meant by thermal analogy of power transistors?
The heat dissipation problem is very much analogous to a
simple
electric circuit and the ohms law. An electric current flows
when there exists a potential difference while the heat flows when
there exists a temperature difference (T2-T1).
32.Write the Thermal-electric analogy parameters.
The following are the thermal-electric analogy parameters.
Tj = Junction temperature TC = Case temperature TA = Ambient
temperature QjA = Total thermal resistance QjC= Transistor thermal
resistance. QCS= Insulator thermal resistance. QSA=Heat sink
thermal resistance.
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UNIT V 1. What is a power supply?
Equipment, which converts the alternating waveform from the
power lines into an essentially direct voltage, is known as power
supply.
2. What are all the subsystems in a power supply?
A power supply consists of following three subsystems. i.
Rectifier.
ii. Filter
iii. Voltage regulator.
3. What is the function of rectifier?
Rectifier is capable of converting a sinusoidal input waveform.
Its average value is zero, into a unidirectional waveform, with a
non-zero
average component. 4. What is rectifier?
Any electrical device which offers a low resistance to the
current in
one direction but a high resistance to the current in the
opposite direction
is called a rectifier. The rectifying device is usually a
semiconductor diode. 5. What is half wave rectifier?
The rectifier circuit which converts only the positive half
cycle of the AC input voltage input voltage into useful DC output
voltage is known as half-wave rectifier.
6. Write down the average DC voltage across the load in a
half-wave
rectifier circuit. The average DC voltage is given by, Vdc
= IdcRL = Vm/{ (1+Rf/RL)
Rf= Diode forward resistance
RL= Load resistance Vm= Maximum amplitude or peak amplitude of
the sinusoidal AC input voltage
7. Define ripple factor.
A measure of the purity of the DC output of a rectifier circuit
is
called the ripple factor r and is defined as,
r = RMS value of AC components of wave Average value of wave
8. What is meant by peak inverse voltage?
The maximum reverse voltage capability of a diode is known
as
peak inverse voltage. 9.What is filter circuit?
Filter circuits are used to reduce the rectifier output ripple.
Either
bypassing the AC output components around the load by a shunt
capacitance or limiting this magnitude to a low value in the load
by a series
inductance or a combination of these two for more efficient
circuits
achieves this. 10. What are all the different types of
filters?
-
The different types of filters are, i. Capacitor filter ii.
Series inductor filter iii. LC filters
1. Capacitance input filter 2. Inductance input filter. iv.
RC filters. 11.How ripples are minimized in the capacitor
filters?
In these types of filters a high value of capacitor is placed
across
directly to the load resistor. This capacitor gets charged
during the conduction period of the rectifier and when Vm decreases
C gets
discharged through RL with a time constant CRL. The capacitor
offers only
low impedance of 1/2 fc. this ripple component of current gets
bypassed
through C 12.What are all the advantages and disadvantages of
capacitor filters?
The advantages of capacitor filters are, At
high loads, i. Small ripple voltage, ii. High output
voltage.
The disadvantages are, i. Poor regulation.
ii. High peak diode current. 13.Why we go for LC filter?
The simple shunt capacitor filter reduces ripple voltage but
increase
the current through the diode. This large current may damage the
diode.
The simple L filter reduces both peak value of output current
and output voltage. So we go for LC filters. This LC filters causes
enough removal of
ripple and restriction on the diode current. 14.What is voltage
multiplier circuit?
Voltage multiplier circuits have the capable of delivering a DC
voltage two or more times the peak value Vm of the applied AC
voltage.
15.Name the commonly used voltage multiplier circuits
The commonly used voltage multiplier circuits are,
i. Voltage doublers ii. Voltage quadruples
iii. Voltage tripler 16.What does voltage regulator mean?
An electron, which keeps the output voltage constant
irrespective of the variation in load current, line voltage and
temperature is known as
electronic voltage regulator. 17.Which characteristic of the
zener diode is useful for voltage regulation?
The reverse bias characteristic of zener diode is useful in
voltage regulation. Since in the reverse biased condition, at the
breakdown voltage Vz, its reverse voltage remains constant for a
large variation of the reverse
current.
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18.What are the factors that define the safe operating area
(SOA) of the transistor? The factors which define the safe
operating area (SOA) of the transistor
are, i. Maximum collector current Ic max rating. ii. Maximum
collector to emitter voltage Vce max rating iii. Maximum power
dissipation PD max rating iv. Maximum junction temperature v.
Second breakdown
19.What is meant by second breakdown in a transistor?
The second breakdown is a phenomenon which results from
the formation of hot spots within the transistor that causes
unbalanced sharing of the total load among the different parts
of the transistor.